Perovskite-type transition metal oxides of an intensely correlated electronic material, of which typical examples are high temperature superconductors and colossal magneto-resistance materials, have such characteristics that the electrical, magnetic and optical properties dramatically change when the charge concentration slightly changes or there is stimulation from an extremely small external field (magnetic field, electrical field, light or the like), and therefore, active research for development of elements using these change properties has been conducted.
The discovery of a phenomenon (colossal magneto-resistance) where electrical resistivity changes over many digits together with metal-insulator transition, which is a phenomenon of magnetic field melting of a charge ordered phase in perovskite-type Mn oxides, which can be represented by R1-xAxMnO3 (R represents a rare earth and a solid solution of one or more types; A represents an alkaline earth metal and a solid solution of one or more types of alkaline earth metals) in the continuing process of design and development of related substances has spurred more and more research. In terms of the properties of this metal-insulator transition, it has been reported in Non-Patent Document 4 that the insulating properties and the properties of the metal-insulator transition of perovskite-type Mn oxides which can be represented by R1-xAxMnO3 are determined by the average ionic radius of R. In addition, it has also been reported that the bandwidth becomes smaller as the average ionic radius of R becomes smaller, and the phase of the insulator relating to the charge ordered phase becomes more stable, and thus the insulating properties improve.
Colossal electro-resistance (hereinafter referred to as CER) changing effects where change in the electrical resistivity spans over many digits were discovered as a type of electrical field and current induced melting phenomenon of a charge ordered phase in a perovskite-type Mn oxide system, such as Pr0.7Ca0.3MnO3, and a memory element using CER effects is introduced in Patent Document 1.
In addition, Patent Document 2 introduces a nonvolatile memory element having a structure where a perovskite-type transition metal oxide having CER effects as Pr0.7Ca0.3MnO3, is used for a semiconductor switch layer, and this semiconductor switch layer is sandwiched between metal electrodes. A resistance random access memory (hereinafter referred to as RRAM) formed of nonvolatile memory elements using this perovskite-type transition metal oxide is characterized by high speed operation, low power consumption, nondestructive readout and the like, in addition to nonvolatile properties, and therefore, expected to substitute DRAM's, SRAM's, flash memories and the like as a universal memory, and thus, development thereof has been progressing.
Furthermore, it was recently reported, as in Non-Patent Documents 1 and 3, that CER effects result from the junction interface between perovskite-type Mn oxide semiconductors, such as Pr0.7Ca0.3MnO3 and other metal materials, such as Ag and Ti. It has been reported, in terms of nonvolatile memory elements using CER effects resulting from the junction interface between such a perovskite Mn oxide semiconductor and a metal, that the resistance in the junction interface reversibly changes between a high resistance state and a low resistance state when an electrical field of a different polarity is applied to the element.
FIG. 1 shows the structure of a conventional memory element made of a Ti/Pr0.7Ca0.3MnO3/SrRuO3 junction having a structure where only one perovskite-type Mn oxide semiconductor layer is sandwiched between metal electrodes, wherein the junction between Pr0.7Ca0.3MnO3 and SrRuO3 provides ohmic contact and CER effects resulting from a non-ohmic contact junction between Ti and Pr0.7Ca0.3MnO3 are gained in a nonvolatile memory.
In the conventional element structure, however, though nonvolatile memory effects resulting from change in the resistance can be gained, the degree of change in the resistance is small, and in addition, the switching properties are poor, and thus, the properties of change in the resistance of the element cannot be controlled.
Patent Document 1: Japanese Unexamined Patent Publication No. H10 (1998)-255481
Patent Document 2: Japanese Unexamined Patent Publication No. 2003-338607
Patent Document 3: U.S. Pat. No. 6,753,561
Non-Patent Document 1: Appl. Phys. Lett. Vol. 83, No. 5, p. 957 (2003)
Non-Patent Document 2: Appl. Phys. Lett. Vol. 85, No. 2, p. 317 (2004)
Non-Patent Document 3: Appl. Phys. Lett. Vol. 85, No. 18, p. 4073 (2004)
Non-Patent Document 4: Phys. Rev. B Vol. 68, No. 9, p. 094417 (2003)